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Say goodbye Trycatch Hell

Ivan Zaldivar on September 18, 2021

Hi everyone! It is possible that it is here for the post title, and I daresay you have had problems handling errors with async/await. In this post ...

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Oyster Lee • Sep 19 '21

This is over-engineered to me

Pedro M. M. • Sep 20 '21 • Edited

Actually, this error tuple-pattern is very common in Go.

if x, err := doSomething(); err != nil {
   // failed!
}
// worked! Safe to use x.

x, err := doSomething()
if err != nil {
  // failed!
}
// worked! Safe to use x

x, err := doSomething(); err == nil {
  // Do something with x only if doSomething() returns no errors
}

if x, ok := booleanOp(); ok {
    // Do something with x only if ok = true
}

In terms of error handling, my favorite is rust because it has less overhead. But it's uglier and a little bit more complicated.

Oyster Lee • Sep 20 '21

Yes, I am a gopher, but do you think over 50% JavaScript developer using Go?

Pedro M. M. • Sep 21 '21 • Edited

No! Of course not lol.

My point was — If a whole modern language, which is known as an easy-to-read language, uses a pattern like that as standard good practice maybe the pattern is not that over-engineered

Ivan Zaldivar • Sep 20 '21

It seems like this at first glance, but it will be useful when the use of trycatch becomes repetitive, and these functions allow you to always reuse them.

Sanna Jammeh • Sep 19 '21

It's a great solution, but it can be made even simpler. You will need another if statement to handle errors in your example. Why not just check if the promise is an error instance.

const res = await (promise).catch((e) => e).

If(res instanceof Error) {
// Handle err
}

Ivan Zaldivar • Sep 20 '21

Mm... I've never thought about it, I like your solution. Thanks!

Szymon Dziewoński • Dec 30 '21

this exactly how I catch errors, and I was wondering whole time reading article "why is there a problem?" However I like wrapper util :)

Kristiqn Tachev • Sep 20 '21 • Edited

You need to catch exceptions on the top level of the function and try not to use the try catch scope until you need to re throw something specific.

With the following approach we are hiding unhandled rejections by providing status code 500 (Database errors for example) and only the correct thrown exceptions will be shown as a result of the requester.

import { randomBytes } from "crypto";

const generateUID = () => randomBytes(8).toString("hex");

/* @HttpCodes
 * Partial list of http codes
 */
export enum HttpCodes {
  Ok = 200,
  Redirect = 302,
  BadRequest = 400,
  Unauthorized = 401,
  PaymentRequired = 402,
  Forbidden = 403,
  NotFound = 404,
  MethodNotAllowed = 405,
  NotAcceptable = 406,
  ProxyAuthenticationRequired = 407,
  RequestTimeout = 408,
  Conflict = 409,
  Gone = 410,
  LengthRequired = 411,
  PreconditionFailed = 412,
  PayloadTooLarge = 413,
  URITooLong = 414,
  UnsupportedMediaType = 415,
  RangeNotSatisfiable = 416,
  ExpectationFailed = 417,
  ImATeapot = 418,
  MisdirectedRequest = 421,
  UnprocessableEntity = 422,
  Locked = 423,
  FailedDependency = 424,
  UnorderedCollection = 425,
  UpgradeRequired = 426,
  PreconditionRequired = 428,
  TooManyRequests = 429,
  RequestHeaderFieldsTooLarge = 431,
  UnavailableForLegalReasons = 451,
  InternalServerError = 500,
  NotImplemented = 501,
  BadGateway = 502,
  ServiceUnavailable = 503,
  GatewayTimeout = 504,
  HTTPVersionNotSupported = 505,
  VariantAlsoNegotiates = 506,
  InsufficientStorage = 507,
  LoopDetected = 508,
  BandwidthLimitExceeded = 509,
  NotExtended = 510,
  NetworkAuthenticationRequired = 511,
}

/* @HttpError
 * Main generic http status error handler
 */
export class HttpError extends Error {
  public reason?: string;
  public isCustomError? = true;
  constructor(
    public readonly message: string | never,
    public readonly statusCode: HttpCodes,
    public readonly details?: object,
  ) {
    super(message);
    this.name = HttpCodes[statusCode];
    this.message = message;

    /* When message is different than status text we have a custom message reason */
    if (HttpCodes[statusCode] !== message) {
      this.reason = message;
    }
  }
}

/* @HttpCodeEnumType
 * Generic enumerable for strict checking of all http codes
 * example: throw HttpErrors.Unauthorized()
 */
export type HttpCodeEnumType<T, K> = {
  [key in keyof typeof HttpCodes]: (
    message?: T,
    details?: object | undefined,
  ) => K;
};

/* @HttpErrors
 * Record of httpErrors
 * example: throw HttpErrors.Unauthorized()
 */
export const HttpErrors: HttpCodeEnumType<string, HttpError> = Object.values(
  HttpCodes,
)
  .filter(k => typeof HttpCodes[k as never] === 'number')
  .reduce((codes, code) => ({
      ...codes,
      [code]: (message = code as never, details: never) =>
        new HttpError(message, HttpCodes[code as never] as never, details),
    }), {} as HttpCodeEnumType<string, HttpError>);


async function myAsyncFunctionNext() {
  return 1;
}
async function myAsyncFunctionNext2() {
  return 2;
}
async function myAsyncFunctionNext3() {
  throw HttpErrors.BadGateway()
  return 3;
}

async function myAsyncFunctionNext4() {
  throw new Error('unauthorized')
}


async function myAsyncFunction() {
  await myAsyncFunctionNext();
  await myAsyncFunctionNext2();
  await myAsyncFunctionNext3();
  // Only try catch if you really want to change the error from specific message thrown
  try {
    await myAsyncFunctionNext4()
  } catch (e) {
    if (e.message === 'unauthorized') {
      throw HttpErrors.Unauthorized()
    }
    throw e;
  }
  return 'successs';
}

export async function handler() {
  try {
    return {
      statusCode: 200,
      body: await myAsyncFunction()
    }
  } catch (e) {
    if (e.isCustomError) {
      return {
        statusCode: e.statusCode,
        body: JSON.stringify(e),
      };
    }

    const uuid = [generateUID(), "-", generateUID(), "-", generateUID()].join(
      ""
    );
    console.error(`[${uuid}]:`, e);
    return {
      statusCode: 500,
      body: JSON.stringify({
        name: "InternalServerError",
        details: { uuid },
      }),
    };
  }
}

Alex Lohr • Sep 22 '21

There's a simple solution. Whatever you await should usually return a promise and thus can be directly caught:

// Don't
try {
  await myAsyncFn();
} catch(e) {
  console.error(e);
}

// Do
await myAsyncFn().catch(console.error);

Anton Trofimenko • Sep 22 '21 • Edited

Done years ago of.js.org

Laurent Perroteau • Sep 22 '21

Another exemple : await-to-js that returns the error first to enforce it to be handled :

const [error, result] = await to(getList());

if (error) ...

Daniel Hillmann • Dec 14 '21 • Edited

Was about to point out it all seemed a bit like re-inventing the wheel, considering modules like this already exist.

I personally use and like await-to-js for many use cases.

Ivan Zaldivar • Sep 22 '21

I like, but is not necessary why is Typescript being used (as long as the developer uses static typing)

Ivan Dilber • Sep 23 '21

Completely pointless refactor exercise IMHO, as now instead of try/catch-ing you need to check each time if a function returned an error or not.

When an exception is thrown it will jump out of all blocks, so there's no need to catch it at the same level. You can set just a single try/catch block on the parent (or even higher) scope to catch all of them.

Gers2017 • Sep 20 '21 • Edited

I usually do

async function handleRequest(req: Response, res: Request){
   const {user, error} = await getUserData(1);
   if(error){
       return res.status(400).json({error})
   }
   res.status(200).json({user})
}

interface UserData{
    name:string
    age: number
}

type UserResponse = { user: UserData | null, error: string | null }
async function getUserData(id: number): Promise<UserResponse>{
    if(id <= 0){
        return {
            user: null,
            error: "Wrong, id can't be less or equal to zero!!!" // Why this failed
        }
    }
    return {
        user: {name: "Jimmy", age: 22},
        error: null
    }
}

Or I'd just throw an error inside the getUserData function

async function handleRequest(){
    try{
        const { user } = await getUserData(1);
        const { user: user2 } = await getUserData(1);
        console.log(user, user2);
    }
    catch(e){
        console.error(e);
    }
}

interface UserData{
    name:string
    age: number
}

type UserResponse = { user: UserData | null }
async function getUserData(id: number): Promise<UserResponse>{
    if(id <= 0){
        throw Error("Wrong, id can't be less or equal to zero!!!")
    } else {
        return { user: {name: "Jimmy", age: 22} }
    }
}

Ivan Zaldivar • Sep 20 '21

Thank you for your contribution, my friend.

Personally, I do not like to handle errors that way, I consider them incorrect, I leave you the link so you can take a look about making these exceptions: developer.mozilla.org/docs/Web/Jav...

async function getUser(id: number): Promise<User> {
  // We throw an exception.
  if (id <= 0) throw new Error("Wrong, id can't be less or equal to zero!!!");
  // Result.
  return { id, email: "abc@domain.com" };
}

Now we handle it.

async function handleRequest(req: Request, res: Response) {
  const [user, err] = await promHandler(getUser(Number(req.params.id)));
  if (!user || err) return res.status(400).json({
    name: "BadRequest",
    message: err.message
  });
  res.status(200).json(user);
}

Gers2017 • Sep 21 '21

Sure, I actually like the golang pattern because is easy to read.
Also I got a different approach by asking about this.

type Result<T> = 
{ result: T, error?: undefined } ||  
{ result?: undefined, error: Error }

async function getUserData(id: number): Promise<Result<UserData>>{
    if(id <= 0){
        return {
            error: new Error("Wrong, id can't be less or equal to zero!!!");
        }
    }
    return {
        result: {name: "Jimmy", age: 22},
    }
}

vnues • Sep 23 '21 • Edited

This is too complicated, I usually do this

const to = (promise: any) => {
  if (!promise) {
    return new Promise((resolve, reject) => {
      reject(new Error('requires promises as the param'))
    }).catch((err: Error) => {
      return [err, null]
    })
  }
  return promise
    .then(function() {
      const arr = Array.from(arguments)
      return [null, ...arr]
    })
    .catch((err: Error) => {
      return [err, null]
    })
}

export const exec = async (sql: string) => {
  const [err, res] = await to(db.exec(sql))
  if (err) {
    throw new Error(err)
  }
  return res
}

// ...async function 
const [data,err] = await to(getUsers())

Because you have to know, promsie comes with try/catch

Валерий Кулаков • Sep 26 '21

First, you have not avoided the problem, because now You have to write some if (error) in the code, and it will be even worse.
Moreover, in fact, we most likely need a global error handler.

Lucian I. Last • Sep 19 '21

Starts to look a lot like GoLang, nice

Although I find your handler example to be a bit over complicated.

Adi Sahar • Sep 20 '21

This matter has been discussed many times. It's better to return the error as the first element to coerce consumers into always handling errors, otherwise it's easy to forget.

Ivan Zaldivar • Sep 20 '21

In this case, it will be hard to forget, since Typescript is used, so you will know that it is the answer or a null value. Thank you for your comments.

João Gonçalves • Sep 22 '21

github.com/supermacro/neverthrow is something to look at, not only avoids the trycatch hell but it also provides a way to easily compose computations!

Ivan Zaldivar • Sep 23 '21

I've been going through the package and it looks interesting. Thanks for your contribution.

Constantin • Sep 20 '21

Love it, in golang people discuss if they want exceptions or not, in Javascript you have them and can decide to just not to.

Federico Kereki • Mar 10 '23

This is going in the direction of Maybe or Either monads.

Rishi U • Sep 23 '21

you lost me at interfaces, not that i didn't understand, but just lost me.

Nikita Makhov • Sep 22 '21 • Edited

Goodbye trycatch hell, hello Promise-async-await hell...

cyrstron • Sep 23 '21

Not sure about golang, but you don't need this many try/catch in JS. Just handle an error only where you need to handle it. Otherwise let it go to a concerned parent.

AbraaoAlves • Sep 20 '21

This is very similar a post on f# blog some years ago ... fsharpforfunandprofit.com/rop/